the genes of the Iroquois complex (Iro-C) araucan (ara),
caupolican (caup), and mirror (mirr), which
encode highly related homeodomain proteins that are members of the prepattern
that controls proneural and provein genes (Go´mez-Skarmeta et
al. 1996; McNeill et al. 1997), are candidates to perform a basic function
in the formation of the notum. Their earliest expression in the wing
disc is restricted to the notum territory, and their absence in clones
of cells induces malformations in the notum but only relatively minor
defects in the patterning of the wing (Go´mez-Skarmeta et al.
1996; Leyns et al. 1996).

In vn mutants, expression of Caup/Ara is lost
(Fig. 2E) and loss of EGFR signaling, in EGFRts clones,
in the medial notum resulted in a loss of Caup/Ara expression
(Fig. 2G). However, clones in the lateral notum continued to express
Caup/Ara (Fig. 2G), suggesting
other factors regulate Iro-C gene expression in these cells at this
stage (Wang,
2000)

ara and caup expression located at the AP compartment
border are necessary for activation of rho at the presumptive
vein L3 and of ac-sc at the proneural clusters that will give
rise to the sensilla campaniformia associated with vein L3 (Gomez-Skarmeta,
1996)

Physical
interactions

{Physical
interactions}

Transcriptional
Regulation

{Regulation}

Structure

{Structure}

Location (protein
and transcript)

ara and caup are expressed ini two symmetric domains
separated by the DV boundary and located near or at the AP compartment
border (Gomez-Skarmeta, 1996; Leyns, 1996)

In wild-type discs, ara and col expression partly overlapped,
with ara expression extending anteriorly by about three rows
of cells into the region corresponding to presumptive L3 vein (Fig.
4I). (Crozatier,
2002)

Protein
Modifications and Regulation

{Modifications}

Related to

{Related
to}

Mutations

Cells lacking Ara or Caup proteins form wing hinge structures (tegula
and axillary sclerites) instead of notum. Moreover, the mutant cells
impose on neighboring wild-type cells more distal developmental fates,
like lateral notum or wing hinge. These
findings support a tergal phylogenetic origin for the most proximal
part of the wing and provide evidence for a novel pattern organizing
center at the border between the apposed notum (Iro-C-expressing) and
hinge (Iro-C-nonexpressing) cells. This border is not a cell
lineage restriction boundary. (Diez
del Corral, 1999)

At the notum, clones homozygous for the Iro-C deletions Df(3L)iroDFM1
or Df(3L)iroDFM3 and induced during the first and
second larval instars associated with extensive malformations: The notum
cuticle was replaced by a mostly naked, corrugated cuticle with sclerotized
structures. In 52 of 116 cases [Df(3L)iroDFM3 clones], these
structures were clearly identifiable as components of an ectopic wing
hinge, for example, axillary sclerites (Fig. 1A,E) and tegula-like cuticle,
with characteristic bristles and sensilla trichoidea and campaniformia
(Fig. 1B,E). (Diez
del Corral, 1999)

43 of the 52 malformations affected the lateral notum. The ectopic
axillary sclerites were always arranged in a mirror-image disposition
with respect to the extant ones (Fig. 1E). (Diez
del Corral, 1999)

Malformations at more medial regions of the notum, which did not
affect the lateral notum, most frequently contained disorganized
groups of tegula-like sensila trichoidea and campaniformia (7 of
the 52 malformations; data not shown) and, in 2 cases, almost complete
ectopic tegulae (Fig. 1C,D). (Diez
del Corral, 1999)

Malformations reaching the central-most regions of the notum caused
defects in the fusion of heminota, which were separated by an undefined
cuticle (not shown). (Diez
del Corral, 1999)

Clones not associated with malformations appeared in flies irradiated
96120 hr after egg laying (AEL). They developed normally in
the central notum or induced invaginating cuticle vesicles in the
lateral regions (not shown). (Diez
del Corral, 1999)

Moreover, appropriately positioned clones rerouted the notumhinge
fold, and this became continuous with the clone-induced fold (Fig.
2DF). In contrast, the border of clones contacting hinge cells
did not form a fold and had normal, wiggly outlines (Fig. 2E). (Diez
del Corral, 1999)

The enhancer trap line l(2)09261, a hinge and wing marker that
is expressed only weakly in the prospective notum (Figs. 3A,B and 4C),
was strongly derepressed in notum Iro-C-cells (Fig.
3CE). teashirt (tsh), a gene strongly expressed in the
hinge and the lateral-most part of the prospective notum, but weakly
expressed in the more central parts of the notum (Fig. 3F), was also
similarly derepressed in the centrally located Iro-C-clones (Fig. 3G). In contrast, vg, optomotor blind
and nubbin (nub), genes that are expressed primarily in
the wing and minimally or not expressed in the hinge (Fig. 4A,B; Williams
et al. 1991; Ng et al. 1995; Grimm and Pflugfelder 1996), were not expressed
in the notum clones (not shown), consistent with the transformation
of Iro-C- cells toward a wing hinge identity. (Diez
del Corral, 1999)

mutant cells that differentiated as ectopic tegula recruited wild-type
cells to form part of this ectopic structure (Fig. 1D). This evidenced
a change of fate of the wild-type cells from notum to tegula. (Diez
del Corral, 1999)

the Iro-C- clones in the notum territory induced neighboring wild-type
cells to express strongly the l(2)09261 marker (Fig. 3C,E).
Interestingly, the expressing cells were located nearest to the
notopleural (NP) region; consequently, their spatial disposition
with respect to the clone was a mirror-image correlate of the NP
cells expressing the marker with respect to the notum/hinge fold
(Fig. 3C). This nonautonomous effect was weaker in more posterior
clones and was not observed in the postnotum territory (not shown).
(Diez
del Corral, 1999)

Another nonautonomous effect concerned the expression of wg, whose
product accumulates in an A/P band that runs near the dorsocentral
region of the prospective notum (Baker 1988; Phillips and Whittle
1993) but not in the hinge (except for a small domain at the prospective
tegula, Fig. 3H). The Iro-C- cells in the notum, which
did not express wg autonomously (Fig. 3I) (in accordance with their
transformation toward hinge cells), caused their wild-type neighbors
closest to the NP region also not to accumulate Wg protein (Fig.
3J). The nonautonomous repression of wg and activation of l(2)09261
and tsh are consistent with the transformation of these cells toward
NP or other lateral notum cells. (Diez
del Corral, 1999)

Interactions between the Iro-C- and Iro-C+ cells were also revealed
by the ectopic fold that surrounded the notum Iro-C- clones (Fig.
2A), as this fold was formed by mutant and wild-type cells (not
shown). Moreover, similar interactions evidently occurred at borders
where cells with ectopic high levels of Iro proteins confronted
cells without or with minimal levels of them, as ectopic folds also
formed at these borders (Fig. 2B,C). Therefore, the fold separating
the prospective notum and wing hinge (Figs. 2A and 3A,B) is probably
induced in early third instar discs by the juxtaposition of Iro-C
expressing and nonexpressing cells (Fig. 5B). Other folds or grooves
may be similarly induced, as mirr is expressed in the Drosophila
embryo, at the dorsal folds, and at the anterior border of each
segment (McNeill et al. 1997). Interestingly, the Xiro1 and Xiro2
genes, Xenopus homologs of ara and caup (Go´mez-Skarmeta et
al. 1998), are expressed at rhombomeres 1, 3, and 5 and may be involved
in generating their borders (J.L. Go´mez-Skarmeta, unpubl.).
(Diez
del Corral, 1999)

Overexpression
/ Ectopic expression

misexpression of ara cause loss of the wing and duplication
of notum (Fig 2A,B) (Wang,
2000)

UASara, driven in the dorsal compartment by apGAL4
removed the dorsal hinge territory, as defined by the expression
of l(2)09261 and the nonexpression of nub (Fig. 4C,D).
The resulting pharate individuals lacked all dorsal hinge elements
(axillary sclerites and tegula), had strongly reduced and distorted
wings, but ectopic notum structures were not discerned (not shown).
Similar adult phenotypes were observed with UAScaup
or UASmirr transgenes (Go´mez-Skarmeta et al.
1996; McNeill et al. 1997), and phenotypes consistent with these
were also found by using drivers dppdisk1-GAL4 (Staehling-Hampton
et al. 1994) or Gal4 line C-765 (Go´mez-Skarmeta et
al. 1996). Simultaneous expression of UASmirr and either
UASara or UAScaup did not modify the results.
Moreover, imaginal disc cells strongly overexpressing UAScaup
at the wing pouch still expressed vg or nub (not shown).
These results indicate that the Iro proteins cannot impose a notum
fate on every wing disc cell, although, if present, they prevent
the normal development of the wing hinge. (Diez
del Corral, 1999)